Utility companies typically distribute power to customers using a network of cables, transformers, capacitors, overvoltage and overcurrent protective devices, switching stations and switchgear. Switchgear is high voltage (e.g. 5 kV-38 kV) equipment used to distribute and control power distribution. Padmounted or underground switchgear includes an enclosure or container that houses bushings, insulation, a bus bar system, and a collection of active switching elements. The active switching elements may include internal active components, such as a fuse, a switch, or an interrupter and external points of connection, such as bushings, to establish line and load connections to an electrical distribution system. Distribution cables transmit power at high voltages. These cables are typically coupled to the switchgear through the switchgear bushings cable connectors. The bushings, in turn, couple to, or form an integral part of, the active switching elements inside the switchgear. The active switching elements are coupled together by a bus bar system in the switchgear assembly.
Other types of switchgear besides padmounted or underground switchgear include switchgear that is used on an overhead distribution system or used in a vault below grade or within load-rooms inside buildings. Such types of switchgear share similar structural and operational components to padmounted switchgear, but are mounted slightly differently and may be connected differently with for example, bare wires instead of insulated cables. Regardless of the type of switchgear, the active switching elements may be used to open and/or close one or more circuit paths through the switchgear automatically, manually, or remotely. One type of active switching element may be a vacuum switch or interrupter having a movable contact that engages or disengages a fixed contact within a vacuum chamber, often formed in a cylindrical tube or bottle. End caps or plates may be attached to the opposite ends of the bottle, and the fixed contact may be maintained in a stationary manner relative to one of the end caps, while the movable contact is slidable positionable with respect to the other end cap between opened and closed positions with respect to the fixed contact within the bottle. The movable contact may be actuated by an operating mechanism to engage or disengage the movable contact to and from the fixed contact within the vacuum chamber in the bottle.
Known vacuum switch or interrupter devices include a rigid reinforcing structure, such as an epoxy or rigid polymeric molding or casting, encapsulating the bottle. The structure is provided to hold and position the vacuum bottle, typically fabricated from ceramic or glass, and the fixed and movable contacts of the bottle with respect to the operating mechanism. In one such device, an elastomeric sleeve surrounds the bottle, and the sleeve is intended to isolate the bottle from the casting and reduce stress on the vacuum bottle as it is encapsulated within the rigid casting and cured at high temperatures.
It has been found, however, that either the bottle or the casting can nonetheless experience breakage due to thermal, mechanical or electrical, stress as the device is used. The materials used to fabricate the casting and the bottle may have different thermal coefficients of expansion, and heat generated by making (closing the contacts), breaking the circuit (opening the contacts), and interrupting fault currents can be significant, which causes the materials to expand rapidly at different rates. Thermal contraction, when cooling after a manufacturing process such as molding, may also cause thermal stress as the materials contract at different rates. Thermal cycling due to seasonal changes from summer to winter or a daily change from day to night may also produce thermal stress, and the cumulative effects of thermal stress may lead to fatigue and premature failure of the device.
Other known vacuum switch or interrupter devices include elastomeric materials for insulation and shielding purposes. For example, a vacuum bottle may be placed within a rigid wound fiberglass tube. The fixed contact may be secured to one end of the tube and the operating mechanism to the other. A secondary elastomeric filler layer fills a space between the bottle and the tube in an attempt to mechanically isolate the bottle from the rigid tube. The tube assembly, including the bottle and the filler layer, may be placed within an elastomeric housing that provides electrical shielding and insulation for the device.
Despite such efforts to isolate the vacuum bottle from mechanical stress, misalignment of the switch or interrupter devices can nonetheless cause the bottle and/or support structure to break due to mechanical forces associated with opening and closing of the contacts in use. If, for example, an actuator shaft of the operating mechanism is misaligned, however slightly, with the axis of the switch or interrupter device, the bottle, and not the supporting structure for the bottle, can become subject to mechanical loads during opening and closing of the contacts. Depending upon the severity and frequency of such loads, the structural integrity of the bottle can be compromised, and perhaps even destroyed. Loading of the bottle due to misalignment of the bottle with respect to the operating shaft may further cause the switch or interrupter to bind, thereby preventing proper opening and closing of the bottle contacts.
Additionally, some known vacuum switch or interrupter devices are susceptible to slight movement of the bottle with respect to the operating mechanism for the bottle, which presents reliability issues in operation, particularly to those using elastomeric housings. If the bottle is not mounted in a manner that assures the fixed contact end of the bottle is secure and cannot move with respect to the shaft of the operating mechanism, the operating mechanism may not fully open and separate the movable contact from the fixed contact. Alternatively, relative movement between the bottle and the operating mechanism may prevent the operating mechanism from fully closing and engaging the movable contact of the vacuum bottle with respect to the fixed contact. The switch contacts must be fully opened or closed for proper functioning. Further, the switch contacts must be held closed with considerable force applied to the movable contact to hold the movable contact tightly against the fixed contact. If this condition is not met, undesirable arcing conditions may occur between the fixed and movable contacts or the fixed and movable contacts may weld together. Additionally, looseness or play in the mounting of the bottle may contribute to bounce between the contacts as they are closed, and this is detrimental to both the mechanical and electrical interface between the contacts. Bounce can also be a source of stress that weakens the bottle, and may cause the switch contacts to weld together.
In a solid dielectric insulated vacuum switch or interrupter device, insulating layers keep internal conductive elements of the device, which may be energized at either high voltage or electrically grounded, electrically isolated from each other. Furthermore, an external ground shield is sometimes, but not necessarily, provided to maintain outer surfaces of the device at ground potential for safety reasons. This ground shield must also be electrically isolated from the energized components. Electrical isolation between potentials is necessary to prevent faults in the electrical system. There are applications, chiefly on an overhead system where the ground shield may not be required because a physical separation of energized components and ground may provide sufficient electrical isolation. In either case, power interruption to line-side connections of the electrical system fed by the device is prevented. Damage to the device itself or to surrounding equipment is also prevented, and people in the vicinity of the switchgear, including but not limited to maintenance workers and technicians, are protected from hazardous conditions. Providing such insulation in a cost effective manner so as to allow the device to withstand the applied voltage and to isolate the circuit when the switch contacts are in the open position is a challenge.
If the air present within the structure is sufficiently stressed, it may breakdown, resulting in a measurable partial discharge. This breakdown may attack the surrounding insulation, ultimately resulting in failure of the insulation system. Therefore, in addition to the external shields, internal cavities in devices with either an external shield or with internal conductive elements at differing electrical potentials that are in close proximity to each other may be surrounded by rubber shields. Theses shields ensure that any air present within the cavity does not have a voltage gradient across it. Eliminating the possible voltage differential eliminates the electrical stress across the air in the cavity, thereby preventing partial discharge and the resulting insulation degradation.
It is desirable to provide a mounting structure and insulation for vacuum switch or interrupter devices that more capably withstands thermal stress and cycling in use, improves reliability of the switchgear as the contacts are opened and closed, simplifies manufacture and assembly of the devices and associated switchgear, and provides cost advantages in relation to known switch or interrupter devices and associated switchgear.